1. Field of the Invention
This invention relates to methods and apparatus for screen printing. More particularly, the invention relates to a multistation screen printing apparatus having an on-line registration capability that enables printing multiple, precisely registered different color images on a plurality of substrates, such as T-shirts, without requiring trial and error printing of proof images on test stock.
2. Description of Background Art
Screen printing is a widely used method of printing multicolored images onto signs, T-shirts and the like. The process involves first preparing a fine-mesh screen of silk, polyester, or metal by coating it with a photosensitive emulsion, contact printing with a bright light source a photographic image on the emulsion, which thereby hardens the emulsion, and washing away the unexposed emulsion. This preparation process leaves open mesh areas of the screen in the areas where ink is to be permitted to flow through to produce a desired image.
A screen bearing an image produced as described above is placed in intimate contact with the surface of the object to be printed. Ink is then poured over the screen and squeezed through the fine openings in the screen by drawing a squeegee across the surface of the screen, to print the desired image on the surface of the object.
It a single color image is to be printed, the printing process described above is a relatively straight forward operation. Thus, when a single color image is printed on an object, a slight misalignment between the object and the screen printing frame results in a printed image which is slightly misaligned with respect to the perimeter of the object. However, a slight misalignment of the printed image is often imperceptible to most people, and therefore is usually acceptable from an appearance standpoint.
When successive, different colored images are printed onto a surface of an object, the orientation of the object with respect to each screen printing frame used to print each successive color becomes much more critical than for single color printing. This is because improper registration between the various colored images making up a composite, multi-colored image results in an aesthetically unacceptable, blurred, smeared appearance.
To produce accurate registration of successive printed images on a given object, screen printers often use a pair of target markings on the screen. A typical target marking is a small circle having perpendicular cross hairs intersecting at the center of the circle. Targets are contained on each screen, and printed on the object with each color. Before each successive color is printed, targets on the screen are visually aligned with the previously printed targets on the object. Target alignment is done before ink is squeezed through the screen to produce the image. This is a time consuming process, and has the additional draw-back of requiring target markings on the printed objects.
A Printing Screen Registering Device employing the method of multiple image registration described above is disclosed in U.S. Pat. No. 2,796,831, issued to P. I. Heestand for a device of that title. The Heestand patent discloses a clamp apparatus for registering, via marks on the screen and sheet to be printed, a printing screen on a table or bed plate for repeatedly printing a pattern in exactly the same registered position on successive sheets of paper or the like.
A screen printing machine using a transparent sheet to check registration of a successive image with a previously printed image is disclosed in German Patent No. 2,816,573, issued Nov. 2, 1978 to Svecio Silkscreen. The machine disclosed includes an adjustable printing table by which an image on an attached sheet may be brought into registration with an image printed on a transparent sheet. Each time a new sheet to be printed is mounted to the printing table, the registration adjustment process must be repeated.
Other screen printing machines relating generally to the field of the present invention are disclosed in the following United States patents:
Harpold, 4,315,461, Feb. 16, 1982, Screen Printing Machine: Discloses a screen printing machine comprising a platen connected with a frame, and a printing head rotatably mounted on the frame and having at least two printing screens pivotally connected therewith for pivoting the same between a printing position, and a raised storage position. The printing head is slidably and removably connected with the frame for selectively locating a pattern on the article to be printed, and/or bodily removing the printing head for replacement with an alternate printing head assembly having a different pattern therein, without disturbing the registry between the mating designs in the printing screens.
Padula, 4,949,635, Aug. 21, 1990, Textile Printing Apparatus: Discloses a textile printing apparatus for multicolor printing that has a plurality of upper and lower arms radiating from a central hub and axle and which are rotatable with respect to one another. The lower arms have platforms for receiving textile workpieces, and the upper arms have clamping means for holding a stencil on top of a workpiece. The upper arms may be raised and lowered to facilitate sequential printing of colors onto a workpiece. Adjustment means are provided for the upper arms so that precise alignment of the stencils can be readily achieved when setting up the apparatus.
Barlow, 4,972,773, Nov. 27, 1990, Registration System For Silk Screen: Discloses a registration system to enable accurate registration of a silk screen in relation to a printing surface which enables adjustment of the silk screen in a simple and efficient manner. The registration system includes support clamps for the silk screen frame together with mounting means for the support clamps which can be easily adjusted by rotating a single, large and readily accessible knob with the mounting structure for the silk screen being counterbalanced by an eccentric weight with the features of the registration system cooperating to enable accurate registration of the silk screen during multiple color printing operations.
Andersen, et al., 4,974,508, Dec. 4, 1990, Screen Printing Apparatus: Discloses a multiple screen printing apparatus for multiple simultaneous printing in which the screen printing carrier head and the platen carrier head are indexed and locked together when the printing screens are down so as to provide precise registration. This indexing and locking arrangement resists degradation of the screen's registration during normal field use. Also, the apparatus has a precise microregistration adjustment system that permits adjustment of the printing screen attachment or screen clamping member relative to the screen carrier head by a controlled and guided adjustment movement in a single plane between a clamping plate and base member. This arrangement provides a coplanar locking so that no movement of the registration occurs during the clamping action.
Harpold, et al., 5,020,430, Jun. 4, 1991, Printer: Discloses a silk screen printing machine having a track about which the printing screens and platens are free to be moved relative to one another in the course of a multi-step printing process. An embodiment is disclosed having the printing screens supported by spokes and platens manually rotatable about a circular supporting track without supporting spokes. The platens move on a common horizontal plane below the printing screens which are brought down onto the platens for screen printing.
Taylor, 5,094,161, Mar. 10, 1992, Counter Top Multi-Color Single Station Printing Method and Apparatus: Discloses a multicolor printing apparatus specifically adapted for printing designs on clothing by the application of ink to successive silk screens, the apparatus including a pedestal having registration pins for insertion in openings in the silk screen supporting frame, and a clamping plate is hingedly mounted above the supporting frame with a lever arm to force the clamping plate downwardly against the support frame into complete registry with the pins. Lift tabs are also provided to lift the supporting frame away from pins after completion of each printing stage, and a printboard is adjustably mounted on a pivot arm to align the article of clothing with respect to the silk screen.
In U.S. Pat. No. 4,846,058, Farris, Multiple Registered Screen Printing Method and Apparatus With Removable Platens, a method and apparatus for screen printing a sequence of accurately registered images on a plurality of sheets or objects such as T-shirts is disclosed. The apparatus includes primary register platens supported by a platen support table. The primary register platen may be moved in a horizontal plane relative to the platen support table, and repositionably secured thereto. Each object to be printed is temporarily fastened to a separate secondary register platen, which may be quickly placed in a precisely repeatable overlying relationship to the primary register platen, and quickly removed therefrom. The apparatus includes a transparent register plate which may be quickly placed in an overlaying relationship to an object on a secondary register platen, in a precisely repeatable position relative to said platen support table, and quickly removed therefrom. A screen frame pivotable in a vertical plane and fastened to the platen support table is pivoted down into overlying contact with the upper surface of the transparent register plate, and a test image printed on the surface. The screen frame is then tilted away, permitting repositioning and securing the primary register platen to align the image with a desired printing position on the surface of the object affixed to a secondary register platen attached to the primary support platen. The transparent register plate is then removed, permitting the surface of the object to be printed. Since each secondary platen bearing an object is removably installable on the apparatus in a precisely repeatable position, each object may be printed with a precisely registered image. Each different successive color image (typically a total of 4 to 6) requires a re-adjustment of the primary register platen with respect to the platen support table, after a test printing of the new color image on the transparent registration plate.
The present invention was conceived of to provide an improved, multi-station screen printing apparatus having an on-line registration capability that enables a plurality of objects to be printed with a sequence of precisely registered, different color images, and which does not require trial and error test printing on test stock to effect precise registration of the multiple images.